Nokia’s Lumia 900 has done what seemed improbable for a Windows Phone: the black version has topped the charts in Amazon’s “Best Sellers in AT&T” category, with the cyan variant holding down the number-three spot as of this writing. For a device running the least-popular smartphone platform, that’s not half bad.

Those ratings are based on pre-orders, since the device hasn’t yet been released. And the buzz surrounding that pre-order is doubtless the source of some of the device’s early success: while Amazon is known for excellent pricing on some pre-release hardware, it’s still relatively rare to see such a powerful smartphone as the Lumia 900 on sale for less than a dollar on-contract, as Amazon was offering until recently.

That news came on the heels of our report that AT&T would be offering the Lumia 900 for just $99 on-contract, with Wal Mart slashing that price in half for their $49 pre-order. Coupled with early publicity efforts like the Smartphone Beta Test, this device looks to be getting an excellent head-start on sales ahead of its April 8 launch.

Even before any official reviews land, it’s pretty clear that the Lumia 900 is the current new-kid darling of the tech media. In addition to solid specs like LTE, an 8MP camera, and a 4.3″ ClearBlack display, Nokia is bringing something important with its first flagship WP7 phone for the US: boldness in design. It’s a device that’s guaranteed to feel different to users and turn the heads of onlookers. More than that, though: the exclusive apps Nokia brings to the WP7 platform offer real added value to purchasers, much more so than their counterparts from other manufacturers. I talked about this in my previous editorial asking why anyone would pay $100 more for the HTC Titan II over the Lumia 900; with the price disparity now standing at $150 to $200 depending on the retailer, my question becomes even more pointed.

But it’s important not to become lost in all the flurry of adoration. In my opinion, the Lumia 900 is tough to beat if you know you’re in the market for a Windows Phone, but if you’re jumping from another platform, you’ll still want to keep an eye out for two main hurdles Microsoft is still trying to overcome.

App Ecosystem

Microsoft’s App Marketplace, the WP7 equivalent of Apple’s App Store or the Google Play Market, recently surpassed 70,000 titles. That’s not a terrible number standing alone; the Android Market boasted only that many apps when the Droid 2 launched in the fall of 2010. Less than two years later, Android currently claims over 450,000 titles, which offers some perspective on how quickly momentum can develop within an ecosystem under the right conditions. Apple’s iOS, leading the pack, currently offers over 585,000 apps.

So buying into Windows Phone right now means accepting a third-party app selection 15% as large as Android, and only 11% as large as iOS. Of course, it’s not all about volume: some believe that users consider a platform viable if it offers the 20 or 30 most popular apps, rather than every app ever written. Microsoft apparently aims to close this gap by the second half of this year, but there’s no getting around it: more apps equals more versatility. A larger selection means the choice is available to you for more specialized capabilities should you need them. It’s great that I can check in to my workplace on Foursquare on any of the top smartphone platforms, but what if I get there and I need to compare the ripeness of melons? Because, you know that’s my job, for some reason. Or suppose I’m having a paranoid day in the UK, and need to know the numbers on roadside deaths in my area, handily displayed on a Google map, right-stat-now? Yeah, there are apps for that – on the more popular platforms. If I’ve got a Windows Phone in that situation, though, I might very well end up dead of an automobile crash with a truck full of rotten melons all because I didn’t have enough app support.

Absurd examples aside, app counts are becoming dull points of comparison – but they’re still crucial to a platform’s success. Microsoft is obviously working hard on this, as the prompt reversal of Rovio’s decision not to launch a WP7 version of Angry Birds Space suggests. But building that app count and attaining the required momentum takes time, and the Lumia 900 will launch well before it attains maturity.

Display Resolution

Formerly the exclusive domain of spec-heads and showoffs, display resolution has become a very popular metric in the past few years thanks to Apple’s legendary marketing finesse, paired with the very real beauty of the iPhone 4/4S Retina display. Beyond Apple, other high-resolution devices like the Galaxy Nexus and Galaxy Note continue to raise the bar for display quality. As almost every smartphone’s primary input method is the display, deficiencies or advantages between devices are easily and quickly spotted.

From a resolution standpoint, that’s bad news for Windows Phones.

The Lumia 900’s display is 4.3,” larger than the iPhone and a number of other devices. It’s also AMOLED, meaning pixels in black regions of the display are actually “off.” This is nothing new, but compared with the iPhone and most Android devices, the viewing experience in a dark room is far superior. Instead of a dark-grey, glowing zone of faux-darkness, blacks on AMOLED displays simply aren’t there. It’s an amazing effect, showcased perfectly on Windows Phone home screens, as the tiles seem to be floating on the phone’s casing, instead of confined to a visible display region.

Unfortunately, that’s where the superiority ends. The Lumia 900, while breathtakingly different, is still considered a “Chassis 1.5″ design by Microsoft, meaning the display resolution is stuck at 800×480, with 217 pixels-per-inch. Apple’s iPhone 4 and 4S boast 960×640 resolution at 326ppi, results which are much more evident in visual form:

Disclaimer: no Windows Phone display is featured in these comparison shots of iOS devices. Just comparing high- to low-res here.

I don’t have close-up photos of a Lumia 900’s display yet, so I couldn’t provide firsthand comparisons, but they’ll be coming soon. The point is obvious: with over a hundred additional points of pixel density, higher-resolution displays like the iPhone’s and Galaxy Nexus’ trounce the Windows Phone standards. And this isn’t one of the easily-dismissable, invisible points of comparison like RAM or cycles per second or number of cores, all of which are secondary to how a device’s OS actually feels. This is a visually evident difference that makes things like reading small text and viewing details on pictures on the Lumia 900 (and every Windows Phone to date) more difficult, so it has to be acknowledged.

Qualification: would either of these things stop me if I were leaving Android for Windows Phone? Not in a second. My eyes are good, so I don’t care much about resolution, and my six months with Windows phone last fall showed me that I could easily survive with the apps available even then. In fact, it’s taken every ounce of discipline I’ve got to resist the urge to order a Lumia 900 for myself (cyan, if you’re wondering). But as I said before, this is a very popular device already, with few naysayers in the tech community thus far, and it hasn’t even launched yet. An environment like that can very easily become an echo chamber of unwarranted positivity, so it’s important sometimes to take a step back and see what’s not perfect about the new kid on the block.

Are you ordering one? Did you pre-order but now having second thoughts? Were you on the fence, but now you’re on your way to Wal-Mart? Drop us a line in the comments; this is a fun launch for the underdog platform, and we’d love to chat about it.